Aluminum titanate composition being stable at high temperature

ABSTRACT

Aluminum titanate composition being stable at high temperature comprises 1.5 to 20 wt. % of Sn component as SnO2 and/or 0.5 to 10 wt. % of rare earth element component as rare earth oxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aluminum titanate having high strengthwhich is stable at high temperature especially even at high temperaturein a reducing atmosphere.

2. Description of the Prior Arts

The ceramic compositions have various characteristics but havedisadvantages of relatively low thermal shock resistance, whereby theusages are limited.

It has been well known that the most effective method for improving thedisadvantage of low thermal shock resistance is to impart low thermalexpansion for the ceramics. Various proposals have been made. Theceramics having low thermal expansion such as lithium compounds e.g.β-spodumene (Li₂ O.Al₂ O₃.4SiO₂) and aluminum-magnesium silicate(2MgO.2Al₂ O₃.5SiO₂) and aluminum titanate (Al₂ O₃.TiO₂) are used insuitable shape. The β-spodumene and aluminum-magnesium silicate havebeen practically used as ceramics for tablewares and gas burner devices.

However, β-spodumene (melting point of 1430° C.) and aluminum-magnesiumsilicate (melting point of 1470° C.) have low melting point whereby theycan be used for only limited special usages even though they areexcellent materials. Accordingly aluminum titanate (melting point of1860° C.) has been studied as ceramics having low thermal expansionwhich can be used in the applications for high thermal shock resistancesuch as the iron steel manufacture.

The aluminum titanate has anisotropic characteristic in crystallographyand has different coefficients of thermal expansion in crystallographicaxes such as -26 × 10⁻⁷ ° C⁻¹ in the a axis direction; 118 × 10⁻⁷ ° C.⁻¹in the b axis direction; and 194 × 10⁻⁷ ° C⁻¹ in the c axis direction.

The average coefficient of thermal expansion is not so small. However,in the sintered product obtained by bonding aluminum titanate particleshaving anisotropic characteristic, a large thermal stress (tensilestress) is internally formed in the directions of the b axis and the caxis because of the differences of coefficients of thermal expansion inthe directions of crystallographic axes whereby many fine cracks arecaused in the direction perpendicular to the b axis and the c axis so asto release stress. As the result, the coefficient of thermal expansionof the sintered product is mainly dependent upon the coefficient ofthermal expansion in the a axis direction whereby aluminum titanateimparts remarkably low coefficient of thermal expansion. On the otherhand, the strength of aluminum titanate is low because of the internalfine cracks.

The relation of the low thermal expansion and low strength of thealuminum titanate sintered product has considered as mentioned above.

There is the other problem of decomposition of aluminum titanate at hightemperature which prevent practical applications of aluminum titanatesintered products.

The decomposition has been considered that Al⁺³ sites in the aluminumtitanate crystals are remarkably larger than the ionic radius of Al⁺³whereby Al⁺³ are taken out from the sites at high temperature. As theresult, the amount of Al₂ O₃ gradually increases and the coefficient ofthermal expansion gradually increases. The Ti⁺³ formed by reducing Ti⁺⁴are entered into the vacancies formed by taking out Al⁺³. Accordingly,when aluminum titanate is used at high temperature in the reducingatmosphere, the change of the lattices may be easily caused.

It has been studied to overcome the disadvantage of the decomposition athigh temperature which is fatal defect among the above-mentioned twodisadvantages of the lowering of strength and the decomposition at hightemperature.

It has been proposed to inhibit the decomposition of aluminum titanateat high temperature by substituting a part of Al⁺³ sites with Mg⁺², Fe⁺³or Cr⁺³ by solid-solubilizing MgO, Fe₂ O₃ or Cr₂ O₃ into aluminumtitanate (U.S. Pat. No. 2,776,896; Japanese Patent Publication No.26688/1967 and Japanese Unexamined Patent Publication No. 23113/1977).The ionic radius of the ions has only slight larger than the ionicradius of Al⁺³ whereby the effect for inhibiting the decomposition athigh temperature is not enough high.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide aluminum titanatecomposition being stable at high temperature whose decomposition at hightemperature is inhibited.

It is the other object of the present invention to provide aluminumtitanate composition which is stable under inhibiting the decompositionat high temperature in a reducing atmosphere.

The foregoing and other objects of the present invention have beenattained by providing aluminum titanate composition being stable at hightemperature which comprises 1.5 to 20 wt. % of the Sn component as SnO₂and/or 0.5 to 10 Wt. % of the rare earth element component as rare earthoxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

As the results of various studies for overcoming disadvantages of theknown additives for inhibiting the decomposition at high temperature, ithas been found that remarkable effect for inhibiting the decompositionof aluminum titanate could be attained by adding the Sn component and/orthe rare earth element component.

The function and effect of the Sn component and/or the rare earthelement component have not yet clearly understood. However, it isconsidered as follows.

The reason of decomposition of aluminum titanate at high temperature ispresumed that, because of smaller ionic radius of Al⁺³ than the space ofAl⁺³ sites, and intense lattice vibration at high temperature, Al⁺³ canbe easily taken out of its site.

When Sn⁺⁴ and/or the rate earth element ions are solid-solubilized intothe aluminum titanate crystals, the ionic radius of Sn⁺⁴ or the rareearth element ions is larger than the ionic radius of Al⁺³ and the ionicradius of Mg⁺², Fe⁺³ or Cr⁺³, and is just fitted to the sites of theAl⁺³ sites in the aluminum titanate crystals and lattice is slightlydistorted so as the Al⁺³ sites can tightly grasp Al⁺³ ions. Because ofthe above effects, the decomposition at high temperature can beinhibited.

The amount of the Sn component is preferably 1.5 to 20 wt. % as SnO₂(weight percent to aluminum titanate component without impurities)because the effect of the Sn component is not remarkable at the ratio ofless than 1.5 wt. % as SnO₂ and the aluminum titanate is not easilyproduced and the mixture of Al₂ O₃, TiO₂ and SnO₂ is easily formed(according to X-ray analysis) in the conventional fabricating conditionat the ratio of more than 20 wt. % as SnO₂. The object of the presentinvention is not attained. From the economical viewpoint, the amount ofthe Sn component is especially 2 to 10 wt. % as SnO₂. It has been alsofound that the strength of aluminum titanate is increased as theadditional effect of the addition of the Sn component.

The amount of the rare earth element component is preferably 0.5 to 10wt. % as the rare earth oxide, because the effect of the rate earthelement component is not remarkable at the ratio of less than 0.5 wt. %as the rare earth oxide and the resulting aluminum titanate may beporous at the ratio of more than 10 wt. % as the rare earth oxide. Theamount of the rare earth element component is especially 1 to 5 wt. % asthe rare earth oxide.

Suitable rare earth elements include La, Ce and Y.

The Sn component and the rare earth element component can be usuallyadded as oxides thereof. Thus, it is possible to add as chlorides,hydroxide, nitrates, acetates and oxalates thereof. These components canbe added to raw materials such as alumina and titania and also can beadded to clinker of aluminum titanate. When these components are addedas oxides, it is preferable to use powder having particle size of 100mesh pass. When these components are added in the water solublecompound, it is possible to add them in a form of aqueous solutions.

In order to use the aluminum titanate composition in practicalapplications, it is necessary to improve the other disadvantage of lowstrength.

The inventors have found that the addition of SiO₂ component isremarkably effective for the purpose.

The function and effect of the SiO₂ component is not clearly understood.However, it is considered that the effect for increasing strength of thealuminum titanate is attained by synergistic effect of the SiO₂component with the Sn component and the rare earth element component.

Certain effect for increasing strength of the aluminum titanatecomposition can be found by the addition of the Sn component and therare earth component without the addition of the SiO₂ component.However, it is preferable to add the SiO₂ component at the ratio of 2 to13 wt. % preferably 3 to 10 wt. % to the aluminum titanate component.The effect for increasing strength is not substantially found at theratio of less than 2 wt. %. The coefficient of thermal expansion is toohigh at the ratio of more than 13 wt. %. The above-mentioned range issuitable from the practical consideration.

It is preferable to add silica especially silica sand as the SiO₂component. The particle size of silica is preferably 100 mesh pass.

The impurities in the raw materials and the other components added forimproving processability affect to the crystalline structure and theeffect of the present invention may be deteriorated. Accordingly, it ispreferable to decrease the amount of the impurities and/or thesecomponents to be less than 5 wt. %.

It is possible to add other components such as Mg component, Fecomponent or Cr component which amount is preferably less than 5 wt. %(oxide base), if necessary.

The Sn component and the rare earth element component can be added asSnO₂ or the rare earth oxide as well as the Sn metal or the rare earthmetals and the other Sn compounds or the rare earth compounds which canbe converted to oxides thereof in the sintering step.

The particle size of the components can be selected as desired though itis not preferable to use particles having a diameter of larger than150μ.

The Sn component and the rare earth element component can be powderyform as well as a solution, if necessary. The mixture of thesecomponents is molded in suitable shape and it is sintered at 1400° to1700° C. preferably 1450° to 1600° C. to form the aluminum titanatecomposition. The invention will be further illustrated by certainexamples to show the effects of the present invention.

EXAMPLES 1 TO 4 AND REFERENCES 1 TO 3:

The industrial grade powdery oxides of Al₂ O₃, TiO₂, SnO₂, La₂ O₃, CeO₂,Y₂ O₃ and Fe₂ O₃ (200 mesh pass) and SiO₂ (100 mesh pass) were weighedas shown in Table 1 and were blended under pulverizing the mixture by avibration mill for 2 hours. The mixture was granulated and molded by theisostatic press under the pressure of 1000 kg/cm², and the moldedcomposition was sintered at 1530° C. for 4 hours.

The typical characteristics of the sintered products are also shown inTable 1. According to the X ray analysis, the sintered products hadaluminum titanate component as the main component.

The sintered products were used as samples and were repeatedly treatedin a reducing atmosphere at the cycle of 400° ⃡900° C. in each 10minutes intervals for 150 times. The characteristics are also shown inTable 1.

As it is clear from the examples, the decomposition of aluminum titanatewas not found in the sintered products of the present invention.Accordingly, the coefficients of thermal expansion of the sinteredproducts were not changed by the treatment. It is clearly found that thesintered products are stable.

It is also found that the strength of the sintered product is improvedby the synergistic effect of the addition of the SiO₂ component.

                                      Table 1                                     __________________________________________________________________________                   Sample                                                                        Exp. 1                                                                             Exp. 2                                                                             Exp. 3                                                                             Exp. 4                                                                             Ref. 1                                                                             Ref. 2                                                                             Ref. 3                           __________________________________________________________________________    Composition (wt.%)                                                            Al.sub.2 O.sub.3                                                                             53   55   55   55   56   56   50                               TiO.sub.2      37   38   38   38   39   44   43                               SnO.sub.2      5    --   --   --   --   --   --                               La.sub.2 O.sub.3                                                                             --   2    --   --   --   --   --                               CeO.sub.2      --   --   2    --   --   --   --                               Y.sub.2 O.sub.3                                                                              --   --   --   2    --   --   --                               Fe.sub.2 O.sub.3                                                                             --   --   --   --   --   --   2                                SiO.sub.2      5    5    5    5    5    --   5                                After sintering                                                               Coefficient of shrinkage by                                                                  6    6    6    6    6    5    6                                 sintering (%)                                                                Coefficient of thermal                                                                       12×10.sup.-7                                                                 8×10.sup.-7                                                                  10×10.sup. -7                                                                5×10.sup.-7                                                                  12×10.sup.-7                                                                 12×16.sup.-7                                                                 12×10.sup.-7                expansion (° C.sup.-1)                                                 (20 to 1000° C)                                                       Bending strength (room temp.)                                                                330  240  215  190  180  <20  170                               (kg/cm.sup.2)                                                                After cycle treatment                                                         Coefficient of thermal                                                                       12×10.sup.-7                                                                 8×10.sup.-7                                                                  10×10.sup.-7                                                                 5×10.sup.-7                                                                  20×10.sup.-7                                                                 30×10.sup.-7                                                                 15×10.sup.-7                expansion (° C.sup.-1)                                                 (20 to 1000° C)                                                       Bending strength (room temp.)                                                                350  230  220  180  150  <20  160                               (kg/cm.sup.2)                                                                X ray analysis decomposition                                                                 none none none none small                                                                              much none                                                                α-Al.sub.2 O.sub.3                                                           Al.sub.2 O.sub.3                                                              TiO.sub.2                             __________________________________________________________________________

The samples of the sintered products prepared in Examples 1 to 4 andReferences 1 to 3 were tested in a reducing atmosphere on the reactivityto a molten iron.

Each crucible was prepared by forming a cavity having a diameter of 30mm and a height of 20 mm in each sample. An iron ball was put in thecavity and the outer space was filled with carbon. The crucible washeated at 1550° C. for 12 hours.

The results are shown in Table 2. As it is clear from the results ofTable 2, the sintered products of the invention were not substantiallydecomposed and were quite stable in comparison with the other sinteredproducts.

                  Table 2                                                         ______________________________________                                        Coefficient of thermal                                                        expansion (° C.sup.-1)                                                 (20 - 1000° C)                                                                            X ray analysis                                             ______________________________________                                        Exp. 1                                                                              22 × 10.sup.-7                                                                           Mostly aluminum titanate                                                      only small amount of                                                          α-Al.sub.2 O.sub.3                               Exp. 2                                                                              15 × 10.sup.-7                                                                           "                                                      Exp. 3                                                                              13 × 10.sup.-7                                                                           "                                                      Exp. 4                                                                              15 × 10.sup.-7                                                                           "                                                      Ref. 1                                                                              60 × 10.sup.-7                                                                           Completely decomposed                                                         to α-Al.sub.2 O.sub.3 and TiO.sub.2              Ref. 2                                                                              75 × 10.sup.-7                                                                           Completely decomposed                                                         to α-Al.sub.2 O.sub.3 and TiO.sub.2              Ref. 3                                                                              13 × 10.sup.-7                                                                           Mostly aluminum titanate                                                      only small amount of                                                          α-Al.sub.2 O.sub.3                               ______________________________________                                    

After the tests, the molten iron was easily separated from the cruciblewithout adhesion. There was no symptom of the reaction on the contactedsurface. The fact shows that the sintered product of the presentinvention can be used as the substrate contacting with the molten ironin the iron and steel industry.

What is claimed is:
 1. An aluminum titanate composition having a lowcoefficient of thermal expansion (20° to 1000° C.) and which is stableat high temperatures and essentially unaffected by reducing atmospheresat high temperatures which consists essentially of 1.5 to 10 wt. % ofSnO₂, 2-13 wt. % of SiO₂ and aluminum titanate.
 2. The aluminum titanatecomposition of claim 1, which contains 2 to 10 wt. % SnO₂.
 3. Thealuminum titanate composition of claim 1, which contains 3 to 10 wt. %of SiO₂.
 4. An aluminum titanate composition having a low coefficient ofthermal expansion (20° to 1000° C.) and which is stable at hightemperatures and essentially unaffected by reducing atmospheres at hightemperatures which consists essentially of 0.5 to 10 wt. % of a rareearth metal oxide, 2 to 13 wt. % of SiO₂ and aluminum titanate.
 5. Thealuminum titanate composition of claim 4, which contains 1 to 5 wt. % ofthe rare earth metal oxide.
 6. The aluminum titanate composition ofclaim 5, which contains 3 to 10 wt. % of SiO₂.